Proton-conducting membrane and the use thereof

ABSTRACT

The present invention relates to a novel proton-conducting polymer membrane based on aromatic polyazoles which contain sulfonic acid groups and in which the sulfonic acid groups are covalently bound to the aromatic ring of the polymer and which can, owing to their excellent chemical and thermal properties, be used for a variety of purposes. Such materials are particularly useful for the production of polymer electrolyte membranes (PEMs) in PEM fuel cells.

The present invention relates to a novel proton-conducting polymermembrane based on aromatic polyazoles which contain sulfonic acid groupsand in which the sulfonic acid groups are covalently bound to thearomatic ring of the polymer and which can, owing to their excellentchemical and thermal properties, be used for a variety of purposes. Suchmaterials are particularly useful for the production of polymerelectrolyte membranes (PEMs) in PEM fuel cells.

Polymer electrolyte membrane fuel cells (PEMFCs) are based on aproton-conducting polymer membrane as electrolytes, viz. the polymerelectrolyte membrane. A fuel cell in this case comprises a plurality ofindividual membrane electrode units (MEUs) connected in series. This MEUcomprises the PEM which is coated on both sides with electrodes, withthe interface between membrane and electrodes being laden with a noblemetal catalyst, usually platinum. The electrochemical reaction of thefuel takes place over this catalyst at the three-phase boundary (fuelgas/catalyst/polymer electrolyte). Hydrogen-rich fuels such as hydrogen,methanol or natural gas are used as fuels at the anode. Oxygen-rich gas,usually air, is supplied to the opposite cathode side. The chemicalenergy of the fuels is in this way converted directly into electricenergy and heat. Water is formed as reaction product. In thisconfiguration, the PEM performs essential functions. Thus, it has tohave a low permeability for the two fuels so as to act as separator, ithas to have a high proton conductivity as electrolyte and at the sametime has to have a high mechanical, chemical and thermal stability inorder to allow long-term use at temperatures up to 200° C. in a stronglyacidic medium without failure occurring. The cell performance andstability is therefore closely linked to the membrane quality.

Electrolytes employed for the fuel cell are solids such as polymerelectrolyte membranes, ceramic oxides, molten carbonates or liquids suchas phosphoric acid or potassium hydroxide solution. Recently, polymerelectrolyte membranes have attracted attention as electrolytes for fuelcells. In principle, a distinction can be made between 2 categories ofpolymer electrolyte membranes.

The first category encompasses cation-exchange membranes comprising apolymer framework containing covalently bound acid groups, preferablysulfonic acid groups.

The sulfonic acid group is converted into an anion with release of ahydrogen ion and therefore conducts protons. The mobility of the protonand thus the proton conductivity is linked directly to the watercontent. If the membrane dries, e.g. as a result of a high temperature,the conductivity of the membrane and consequently the power of the fuelcell decreases drastically. The operating temperatures of fuel cellscontaining such cation-exchange membranes are thus limited to theboiling point of water. Materials used for polymer electrolyte membranesare thus, for example, perfluorosulfonic acid polymers. Theperfluorosulfonic acid polymer (e.g. Nafion) generally has aperfluorinated hydrocarbon skeleton such as a copolymer oftetrafluoroethylene and trifluorovinyl and a side chain bearing asulfonic acid group, e.g. a side chain bearing a sulfonic acid groupbound to a perfluoroalkyl group. Moistening of the fuel represents agreat technical challenge for the use of polymer electrolyte membranefuel cells (PEMFCs) in which conventional, sulfonated membranes such asNafion are used.

The second category which has been developed encompasses polymerelectrolyte membranes comprising complexes of basic polymers and strongacids. Thus, WO 96/13872 and the corresponding U.S. Pat. No. 5,525,436describe a process for producing a proton-conducting polymer electrolytemembrane, in which a basic polymer, for example a polyazole, is treatedwith a strong acid such as phosphoric acid, sulfuric acid, etc.

Polyazoles such as polybenzimidazoles (®Celazole) have been known for along time. Such polybenzimidazoles (PBIs) are usually prepared byreaction of 3,3′-4,4′-tetraaminobiphenyl with isophthalic acid ordiphenylisophthalic acid or esters thereof in the melt. The prepolymerformed solidifies in the reactor and is subsequently comminutedmechanically. The pulverulent prepolymer is subsequently subjected tofinal polymerization in the solid state at temperatures of up to 400° C.so as to give the desired polybenzimidazole.

To produce polymer films, the PBI is dissolved in polar, aproticsolvents such as dimethylacetamide (DMAc) in a further step and a filmis produced by classical methods.

In a further step, the film of basic polymer or polymer blend isimpregnated or doped with a strong acid, preferably a mineral acid. Forthis purpose, the film of a basic polymer or polymer blend is dippedinto a strong acid, preferably phosphoric acid, so that the film isimpregnated with the strong acid and becomes a proton-conductingmembrane.

J. Electrochem. Soc. Volume 142, No. 7, 1995, pp. L121-L123, describessuch doping of a polybenzimidazole in phosphoric acid.

Proton-conducting, i.e. acid-doped, polyazole membranes for use in PEMfuel cells are accordingly already known. The doped, basic polyazolefilms then act as proton conductors and separators in polymerelectrolyte membrane fuel cells (PEM fuel cells).

Owing to the excellent properties of the polyazole polymer, such polymerelectrolyte membranes can, when converted into membrane-electrode units(MEUs), be used in fuel cells at long-term operating temperatures above100° C., in particular above 120° C. This high long-term operatingtemperature allows the activity of the catalysts based on noble metalswhich are present in the membrane-electrode unit (MEU) to be increased.Particularly when using products from the reforming of hydrocarbons,significant amounts of carbon monoxide are present in the reformer gasand usually has to be removed by means of a complicated gas processingor gas purification step. The possibility of increasing the operatingtemperature enables significantly higher concentrations of CO impuritiesto be tolerated over the long term.

The use of polymer electrolyte membranes based on polyazole polymersfirstly enables the complicated gas processing or gas purification stepto be dispensed with in some cases and secondly enables the catalystloading in the membrane-electrode unit to be reduced. Both areindispensable prerequisites for large-scale use of PEM fuel cells, sinceotherwise the costs of a PEM fuel cell system are too high.

The previously known acid-doped polymer membranes based on polyazolesdisplay a favorable property profile. However, owing to the applicationsdesired for PEM fuel cells, in particular in the automobile sector andin decentralized power and heat generation (stationary sector), theseneed to be improved overall. Furthermore, the previously known polymermembranes have a high content of dimethylacetamide (DMAc) which cannotbe removed completely by means of known drying methods. The Germanpatent application No. 10109829.4 describes a polymer membrane which isbased on polyazoles and in which the DMAc contamination has beeneliminated. Although such polymer membranes display improved mechanicalproperties, specific conductivities do not exceed 0.1 S/cm (at 140° C.).

A significant advantage of such a membrane doped with phosphoric acid isthe fact that this system can be operated at temperatures above 100° C.without the moistening of the fuels which is otherwise necessary. Thisis due to the ability of phosphoric acid to transfer protons withoutaddition of water by means of the “Grotthus mechanism” (K.-D. Kreuer,Chem. Mater. 1996, 8, 610-641). Such a water-free transport mechanism isof particular interest for use in a direct methanol fuel cell. Here, thefuel used is methanol which can be oxidized directly without thenecessity of a preceding reforming step. To achieve the possibility ofwater-free proton transport, methanol is not carried along with themigrating proton in the form of a hydrating shell as is usual in the“vehicle” mechanism (K.-D. Kreuer, Chem. Mater. 1996, 8, 610-641). Thepower and efficiencies of a direct methanol fuel cell can be improved byreducing this methanol “crossover”.

The possibility of operation at temperatures above 100° C. results infurther advantages for the fuel cell system. Firstly, the sensitivity ofthe Pt catalyst to impurities in the gas, in particular CO, is greatlyreduced and the catalytic activity is improved. CO is formed asby-product in the reforming of the hydrogen-rich gas comprisinghydrocarbon compounds, e.g. natural gas, methanol or petroleum spirit,or as intermediate in the direct oxidation of methanol. The CO contentof the fuel typically has to be less than 100 ppm at temperatures of<100° C. However, at temperatures in the range 150-200°, 10 000 ppm ormore of CO can also be tolerated (N. J. Bjerrum et al., Journal ofApplied Electrochemistry, 2001, 31, 773-779). This leads to significantsimplifications of the upstream reforming process and thus to costreductions for the total fuel cell system.

In addition to the abovementioned acid-doped polymer membranes producedby means of intensive processes, a polymer electrolyte membranecomprising a basic polymer can also be produced directly frompolyphosphoric acid. For this purpose, the starting monomers asdescribed in the German patent application No. 10117686.4 or theprepolymers as described in the German patent application No. 10144815.5or the infusible starting polymer as described in the German patentapplication No. 10117687.2 are/is dissolved in polyphosphoric acid andsubsequently spread directly by means of a doctor blade asproton-conducting membrane without subsequent treatment such as drying,washing and doping being necessary. The main advantage of this method isprocess simplification. In addition, new types of polymer electrolytemembrane can be tailor-made by means of targeted selection of themonomers.

Apart from the abovementioned materials, corresponding processes forsulfonating polymers are also known from the prior art.

To produce a PEM from a sulfonated polyether ketone (PEK), the PEKpolymer is firstly dissolved in a suitable solvent, e.g. concentratedsulfuric acid, after which an aggressive sulfonating agent such as oleumor chlorosulfonic acid is added. This sulfonated polymer is separatedfrom the sulfonation solution in a further process step. For furtherprocessing, it then has to be converted into the neutral salt form bymeans of a basic solution in a further step. The polymer is subsequentlybrought back into solution and, in a further process step, a polymerfilm is produced by film casting or spreading by means of a doctorblade. The solvent, preferably N-methylpyrrolidone orN-dimethylacetamide is evaporated by drying. The film then has to betreated with acid again and subsequently washed until neutral. As analternative, a polymer film which has been produced beforehand by meansof extrusion or film casting and subsequently modified by radiationgrafting, e.g. a styrene-modified, partially fluorinated membrane, canbe treated with a sulfonation solution comprising chlorosulfonic acidand an anhydrous solvent, e.g. tetrachloroethane (EP-A-667983,DE-A-19844645).

In these sulfonation processes using very strong sulfonating agents,uncontrolled sulfonation at many places on the polymer takes place. Thesulfonation can also lead to chain rupture and thus to impairment of themechanical properties and finally to premature failure of the fuel cell.

Sulfonated polybenzimidazoles, too, are already known from theliterature. Thus, U.S. Pat. No. 4,634,530 describes a sulfonation of anundoped polybenzimidazole film by means of a sulfonating agent such assulfuric acid or oleum in the temperature range up to 100° C.

Furthermore, Staiti et al. (P. Staiti in J. Membr. Sci. 188 (2001) 71)have described the preparation and properties of sulfonatedpolybenzimidazole. It was in this case not possible to carry out thesulfonation of the polymer in the solution. On addition of thesulfonating agent to the PBI/DMAc solution, the polymer precipitates. Tocarry out the sulfonation, a PBI film was produced first and this wasdipped into a dilute sulfuric acid. The specimens were then treated attemperatures of about 475° C. for 2 minutes to effect sulfonation. Thesulfonated PBI membranes have only a maximum conductivity of 7.5* 10⁻⁵S/cm at a temperature of 160° C. The maximum ion-exchange capacity is0.12-meq/g. It was likewise shown that such sulfonated PBI membranes arenot suitable for use in a fuel cell.

The production of sulfoalkylated PBI membranes by reacting ahydroxyethyl-modified PBI with a sultone is described in U.S. Pat. No.4,997,892. On the basis of this technology, it is possible to producesulfopropylated PBI membranes (Sanui et al. in Polym. Adv. Techn. 11(2000) 544). The proton conductivity of such membranes is 10⁻³ S/cm andis thus too low for use in fuel cells in which 0.1 S/cm is sought.

It is an object of the present invention to provide a sulfonated,high-temperature-stable polymer membrane having a high conductivity evenat high operating temperatures. This object is achieved by the provisionof a polymer electrolyte membrane comprising sulfonated polyazoles dopedwith phosphoric acid. The sulfonation of the polyazole is effected byaddition of a suitable sulfonating agent during or immediately after thepolymerization to produce the polyazoles.

The present invention provides a proton-conducting polymer membranewhich is based on sulfonated polyazoles and is obtainable by a processcomprising the steps

-   A) mixing of one or more aromatic tetraamino compounds with one or    more aromatic carboxylic acids or esters thereof which contain at    least two acid groups per carboxylic acid monomer, or mixing of one    or more aromatic and/or heteroaromatic diaminocarboxylic acids, in a    polyphosphoric acid/sulfonating agent mixture to form a solution    and/or dispersion,-   B) application of a layer using the mixture from step A) to a    support or an electrode,-   C) heating of the sheet-like structure/layer obtainable according to    step B) under inert gas at temperatures of up to 350° C., preferably    up to 280° C., to form the polyazole polymer,-   D) treatment of the membrane formed in step C), preferably until it    is self-supporting.

The aromatic and heteroaromatic tetraamino compounds used according tothe invention are preferably 3,3′,4,4′-tetraaminobiphenyl,2,3,5,6-tetraaminopyridine, 1,2,4,5-tetraaminobenzene,bis(3,4-diaminophenyl) sulfone, bis(3,4-diaminophenyl) ether,3,3′,4,4′-tetraaminobenzophenone, 3,3′,4,4′-tetraaminodiphenylmethaneand 3,3′,4,4′-tetraaminodiphenyldimethylmethane and their salts, inparticular their monohydrochloride, dihydrochloride, trihydrochlorideand tetrahydrochloride derivatives.

The aromatic carboxylic acids using step A) are, in particular,dicarboxylic acids, tricarboxylic acids and tetracarboxylic acids ortheir esters or their anhydrides.

The term aromatic carboxylic acids likewise encompasses heteroaromaticcarboxylic acids. The aromatic dicarboxylic acids are preferablyisophthalic acid, terephthalic acid, phthalic acid, 5-hydroxyisophthalicacid, 4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid,5-aminoisophthalic acid, 5-N,N-dimethylaminoisophthalic acid,5-N,N-diethylaminoisophthalic acid, 2,5-dihydroxyterephthalic acid,2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid,2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid,3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalicacid, 2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-dicarboxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid, 4-carboxycinnamic acid, or their C1-C20-alkyl esters orC5-C12-aryl esters, or their acid anhydrides or acid chlorides. Thearomatic tricarboxylic acids, tetracarboxylic acids or theirC1-C20-alkyl esters or C5-C12-aryl esters or their acid anhydrides ortheir acid chlorides are preferably 1,3,5-benzenetricarboxylic acid(trimesic acid), 1,2,4-benzenetricarboxylic acid (trimellitic acid),(2-carboxyphenyl)iminodiacetic acid, 3,5,3′-biphenyltricarboxylic acid,3,5,4′-biphenyltricarboxylic acid.

The aromatic tetracarboxylic acids or their C1-C20-alkyl esters orC5-C12-aryl esters or their acid anhydrides or their acid chlorides arepreferably 3,5,3′,5′-biphenyltetracarboxylic acid,1,2,4,5-benzenetetracarboxylic acid, benzophenonetetracarboxylic acid,3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid.

The heteroaromatic carboxylic acids used according to the invention areheteroaromatic dicarboxylic acids and tricarboxylic acids andtetracarboxylic acids or their esters or their anhydrides. For thepurposes of the present invention, heteroaromatic carboxylic acids arearomatic systems in which at least one nitrogen, oxygen, sulfur orphosphorus atom is present in the aromatic. Preference is given topyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid,pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid,2,4,6-pyridinetricarboxylic acid, benzimidazole-5,6-dicarboxylic acid,and also their C1-C20-alkyl esters or C5-C12-aryl esters, or their acidanhydrides or their acid chlorides.

The content of tricarboxylic acids or tetracarboxylic acids (based ondicarboxylic acid used) is in the range from 0 to 30 mol %, preferablyfrom 0.1 to 20 mol %, in particular from 0.5 to 10 mol %.

Mixtures of at least 2 different aromatic carboxylic acids arepreferably used in step A). Particular preference is given to usingmixtures comprising not only aromatic carboxylic acids but alsoheteroaromatic carboxylic acids. The mixing ratio of aromatic carboxylicacids to heteroaromatic carboxylic acids is from 1:99 to 99:1,preferably from 1:50 to 50:1.

These mixtures are, in particular, mixtures of N-heteroaromaticdicarboxylic acids and aromatic dicarboxylic acids. Nonlimiting examplesare isophthalic acid, terephthalic acid, phthalic acid,2,5-dihydroxyterephthalic acid, 2,6-dihydroxyiso-phthalic acid,4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-carboxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoro-methylphthalic acid, pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazinedicarboxylic acid.

The aromatic and heteroaromatic diaminocarboxylic acids used accordingto the invention are preferably diaminobenzoic acid and theirmonohydrochloride and dihydrochloride derivatives.

The polyphosphoric acid used in step A) is a commercial polyphosphoricacid as can be obtained, for example, from Riedel-de Haen. Thepolyphosphoric acids H_(n+2)P_(n)O_(3n+1) (n>1) usually have an assaycalculated as P₂O₅ (acidimetric) of at least 83%. In place of a solutionof the monomers, it is also possible to produce a dispersion/suspension.The mixture produced in step A) has a weight ratio of polyphosphoricacid to the sum of all monomers of from 1:10 000 to 10 000:1, preferablyfrom 1:1000 to 1000:1, in particular from 1:100 to 100:1.

The sulfonating agent used in step A) can be i) concentrated sulfuricacid (>95%), ii) chlorosulfonic acid, iii) a complex of SO₃ with a Lewisbase or other organic constituents, iv) an acyl or alkyl sulfate, v) anorganic sulfonic acid or vi) a mixture of i to v.

The amount of sulfonating agent used is from 1 to 20% by weight based onthe polyphosphoric acid, preferably from 2 to 15% by weight and veryparticularly preferably 5-10% by weight.

The layer formation in step B) is carried out by measures known per se(casting, spraying, spreading by doctor blade) known per se from theprior art for polymer film production. As supports, it is possible touse all supports which are inert under the conditions. To adjust theviscosity, the solution can, if appropriate, be admixed with phosphoricacid (concentrated phosphoric acid, 85%). In this way, the viscosity canbe set to the desired value and the formation of the membrane can bemade easier. The layer produced in step B) has a thickness of from 20 to4000 μm, preferably from 30 to 1500 μm, in particular from 50 to 500 μm.

The polyazole-based polymer formed in step C) comprises recurring azoleunits of the general formula (I) and/or (II) and/or (III) and/or (IV)and/or (V) and/or (VI) and/or (VII) and/or (VIII) and/or (IX) and/or (X)and/or (XI) and/or (XII) and/or (XIII) and/or (XIV) and/or (XV) and/or(XVI) and/or (XVI) and/or (XVII) and/or (XVIII) and/or (XIX) and/or (XX)and/or (XXI) and/or (XXII).

where

-   the radicals Ar are identical or different and are each a    tetravalent aromatic or heteroaromatic group which can be monocyclic    or polycyclic,-   the radicals Ar¹ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar² are identical or different and are each a divalent    or trivalent aromatic or heteroaromatic group which can be    monocyclic or polycyclic,-   the radicals Ar³ are identical or different and are each a trivalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁴ are identical or different and are each a trivalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁵ are identical or different and are each a    tetravalent aromatic or heteroaromatic group which can be monocyclic    or polycyclic,-   the radicals Ar⁶ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁷ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁸ are identical or different and are each a trivalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁹ are identical or different and are each a divalent    or trivalent or tetravalent aromatic or heteroaromatic group which    can be monocyclic or polycyclic,-   the radicals Ar¹⁰ are identical or different and are each a divalent    or trivalent aromatic or heteroaromatic group which can be    monocyclic or polycyclic,-   the radicals Ar¹¹ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals X are identical or different and are each oxygen,    sulfur or an amino group which bears a hydrogen atom, a group having    1-20 carbon atoms, preferably a branched or unbranched alkyl or    alkoxy group, or an aryl group as further radical,-   the radicals R are identical or different and are each hydrogen, an    alkyl group or an aromatic group and-   n, m are each an integer greater than or equal to 10, preferably    greater than or equal to 100,    and has a sulfur content of from 2 to 20% by weight (determined by    means of elemental analysis), in particular from 4 to 10% by weight.

Preferred aromatic or heteroaromatic groups are derived from benzene,naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenyl sulfone, quinoline,pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine,tetrazine, pyrrole, pyrazole, anthracene, benzopyrrole, benzotri-azole,benzoxathiadiazole, benzoxadiazole, benzopyridine, benzopyrazine,benzopyrazidine, benzopyrimidine, benzopyrazine, benzotriazine,indolizine, quinolizine, pyridopyridine, imidazopyrimidine,pyrazinopyrimidine, carbazole, acridine, phenazine, benzoquinoline,phenoxazine, phentothiazine, acridizine, benzopteridine, phenanthrolineand phenanthrene, which may also be substituted.

Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can have any substitutionpattern; in the case of phenylene, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰,Ar¹¹ can be, for example, ortho-, meta- or para-phenylene. Particularlypreferred groups are derived from benzene and biphenylene, which mayalso be substituted.

Preferred alkyl groups are short-chain alkyl groups having from 1 to 4carbon atoms, e.g. methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups may be substituted.

Preferred substituents are halogen atoms such as fluorine, amino groups,hydroxy groups or short-chain alkyl groups such as methyl or ethylgroups.

Preference is given to polyazoles having recurring units of the formula(I) in which the radicals X within one recurring unit are identical.

The polyazoles can in principle also have different recurring unitswhich differ, for example, in their radical X. However, preference isgiven to only identical radicals X being present in a recurring unit.

Further, preferred polyazole polymers are polyimidazoles,polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles, poly-(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

In a further embodiment of the present invention, the polymer comprisingrecurring azole units is a copolymer or a blend comprising at least twounits of the formulae (I) to (XXII) which differ from one another. Thepolymers can be in the form of block copolymers (diblock, triblock),random copolymers, periodic copolymers and/or alternating polymers.

In a particularly preferred embodiment of the present invention, thepolymer comprising recurring azole units is a polyazole comprising onlyunits of the formula (I) and/or (II).

The number of recurring azol units in the polymer is preferably greaterthan or equal to 10. Particularly preferred polymers contain at least100 recurring azole units.

For the purposes of the present invention, polymers comprising recurringbenzimidazole units are preferred. Some examples of extremelyadvantageous polymers comprising recurring benzimidazole units arerepresented by the following formulae:

where n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100.

The polyazoles obtainable by means of the process described, but inparticular the polybenzimidazoles, have a high molecular weight.Measured as intrinsic viscosity, it is at least 1.4 dl/g and is thussignificantly above that of commercial polybenzimidazole (IV<1.1 dl/g).

If tricarboxylic acids and/or tetracarboxylic acids are present in themixture obtained in step A), they effect branching/crosslinking of thepolymer formed. This contributes to an improvement in the mechanicalproperties. The polymer layer produced in step C) is treated in thepresence of moisture at temperatures and for a time sufficient for thelayer to have sufficient strength for use in fuel cells. The treatmentcan be carried out until the membrane is self-supporting, so that it canbe detached from the support without damage.

In one variant of the process, the formation of oligomers and/orpolymers can be brought about by heating the mixture from step A) totemperatures of up to 350° C., preferably up to 280° C. Depending on thetemperature and time selected, the heating in step C) may be able to bepartly or entirely omitted. This variant is also provided by the presentinvention.

Furthermore, it has been found that when using aromatic dicarboxylicacids (or heteroaromatic dicarboxylic acids) such as isophthalic acid,terephthalic acid, 2,5-dihydroxyterephthalic acid,4,6-dihydroxyisophthalic acid, 2,6-dihydroxysophthalic acid, diphenicacid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-carboxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid, pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazinedicarboxylic acid, the temperature in step C), or if theformation of oligomers and/or polymers is desired as early as step A),is advantageously in the range up to 300° C., preferably in the rangefrom 100° C. to 250° C.

The treatment of the membrane in step D) is carried out at temperaturesabove 0° C. and less than 150° C., preferably at temperatures in therange from 10° C. to 120° C., in particular from room temperature (20°C.) to 90° C., in the presence of moisture or water and/or steam. Thetreatment is preferably carried out under atmospheric pressure, but canalso be carried out under superatmospheric pressure. It is importantthat the treatment is carried out in the presence of sufficientmoisture, as a result of which the polyphosphoric acid presentcontributes to strengthening of the membrane by partial hydrolysis toform low molecular weight polyphosphoric acid and/or phosphoric acid.

The partial hydrolysis of the polyphosphoric acid in step D) leads tostrengthening of the membrane and to a decrease in the layer thicknessand formation of a membrane having a thickness of from 15 to 3000 μm,preferably from 20 to 2000 μm, in particular from 20 to 1500 μm, whichis self-supporting. The intramolecular and intermolecular structurespresent in the polyphosphoric acid layer formed in step B) lead in stepC) to ordered membrane formation, which is responsible for theparticular properties of the membrane formed.

The upper temperature limit to the treatment in step D) is generally150° C. In the case of extremely brief action of moisture, for exampleof superheated steam, this steam can also be hotter than 150° C. Theduration of the treatment is important in determining the uppertemperature limit.

The partial hydrolysis (step D) can also be carried out in chambershaving a controlled temperature and humidity, in which case thehydrolysis can be controlled in a targeted fashion in the presence of adefined amount of moisture. The humidity can be set to a specific valueby means of the temperature or saturation of the environment in contactwith the membrane, for example gases such as air, nitrogen, carbondioxide or other suitable gases or steam. The treatment time isdependent on the parameters selected above.

The treatment time is also dependent on the thickness of the membrane.

In general, the treatment time ranges from a few seconds to someminutes, for example in the presence of superheated steam, or up toentire days, for example in air at room temperature and relatively lowatmospheric humidity. The treatment time is preferably from 10 secondsto 300 hours, in particular from 1 minute to 200 hours.

If the partial hydrolysis is carried out at room temperature (20° C.) bymeans of ambient air at a relative atmospheric humidity of 40-80%, thetreatment time is in the range from 1 to 200 hours.

The treatment with water in step D) can also be carried out to such adegree that the phosphoric acid is completely removed from the membrane.

The membrane obtained according to step D) can be self-supporting, i.e.it can be detached from the support without damage and subsequently, ifappropriate, be directly used further.

The concentration of phosphoric acid and thus the conductivity of thepolymer membrane of the invention can be set via the degree ofhydrolysis, i.e. the time, temperature and ambient humidity. Accordingto the invention, the concentration of the phosphoric acid is reportedas mole of acid per mole of repeating units of the polymer. For thepurposes of the present invention, a concentration (mole of phosphoricacid per mole of repeating units of the formula (III), i.e.polybenzimidazole) of from 10 to 25, in particular from 12 to 20, ispreferred. Such high degrees of doping (concentrations) can be obtainedonly with great difficulty, if at all, by doping of polyazoles withcommercially available ortho-phosphoric acid.

Subsequent to the treatment according to step D), the membrane can beadditionally crosslinked on the surface by the action of heat in thepresence of atmospheric oxygen. This hardening of the membrane surfaceachieves an additional improvement in the properties of the membrane.

Crosslinking can also be effected by action of IR or NIR (IR-infrared,i.e. light having a wavelength of more than 700 nm; NIR=near IR, i.e.light having a wavelength in the range from about 700 to 2000 nm or anenergy in the range from about 0.6 to 1.75 eV). A further method isirradiation with β-rays. The radiation dose is in the range from 5 to200 kGy.

The polymer membrane of the invention displays improved materialsproperties compared to the previously known doped polymer membranes. Inparticular, it displays improved power compared to known doped polymermembranes. This is due, in particular, to an improved protonconductivity at temperatures above and below 100° C. without moisteningof the membrane. The specific conductivity both at room temperature andat 120° C. is at least 0.06 S/cm, preferably at least 0.08 S/cm, inparticular at least 0.09 S/cm.

To achieve a further improvement in the use properties, fillers, inparticular proton-conducting fillers, and additional acids can also beadded to the membrane. The addition can be carried out either in step A)or after the polymerization.

Nonlimiting examples of proton-conducting fillers are

-   sulfates such as CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄, NaHSO₄,    KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,-   phosphates such as Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃, UO₂PO₄3H₂O,    H₈UO₂PO₄, Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄, LiH₂PO₄, NH₄H₂PO₄,    CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄, H₅Sb₅P₂O₂₀,-   polyacids such as H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW₁₂O₄₀.nH₂O    (n=21-29), H_(x)WO₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃,    HTiNbO₅, HTiTaO₅, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄,-   selenites and arsenides such as (NH₄)₃H(SeO₄)₂, UO₂AsO₄,    (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂, Rb₃H(SeO₄)₂,-   oxides such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃,-   silicates such as zeolites, zeolites(NH₄+), sheet silicates,    framework silicates, H-natrolites, H-mordenites, NH₄-analcines,    NH₄-sodalites, NH₄-gallates, H-montmorillonites,-   acids such as HClO₄, SbF₅,-   fillers such as carbides, in particular SiC, Si₃N₄, fibers, in    particular glass fibers, glass powders and/or polymer fibers,    preferably ones based on polyazoles.

In addition, this membrane can further comprise perfluorinated sulfonicacid additives (0.1-20% by weight, preferably 0.2-15% by weight, veryparticularly preferably 0.2-10% by weight). These additives lead to anincrease in power, in the vicinity of the cathode to an increase in theoxygen solubility and oxygen diffusion and to a reduction in theadsorption of phosphoric acid and phosphate onto platinum. (Electrolyteadditives for phosphoric acid fuel cells. Gang, Xiao; Hjuler, H. A.;Olsen, C.; Berg, R. W.; Bjerrum, N. J. Chem. Dep. A, Tech. Univ.Denmark, Lyngby, Den. J. Electrochem. Soc. (1993), 140(4), 896-902 andPerfluorosulfonimide as an additive in phosphoric acid fuel cell. Razaq,M.; Razaq, A.; Yeager, E.; DesMarteau, Darryl, D.; Singh, S. Case Cent.Electrochem. Sci., Case West, Reserve Univ., Cleveland, Ohio, USA. J.Electrochem. Soc. (1989), 136(2), 385-90.)

Nonlimiting examples of persulfonated additives are:

-   trifluoromethanesulfonic acid, potassium trifluoromethanesulfonate,    sodium trifluoromethanesulfonate, lithium    trifluoromethanesulfontate, ammonium trifluoro-methanesulfonate,    potassium perfluorohexanesulfonate, sodium perfluorohexanesulfonate,    lithium perfluorohexanesulfonate, ammonium perfluorohexanesulfonate,    perfluorohexanesulfonic acid, potassium nonafluorobutanesulfonate,    sodium nonafluorobutanesulfonate, lithium nonafluorobutanesulfonate,    ammonium nonafluorobutanesulfonate, cesium    nonafluorobutanesulfonate, triethylammonium    perfluorohexanesulfonate, perfluorosulfonimides and Nafion.

Furthermore, the membrane can further comprise additives which scavenge(primary antioxidants) or destroy (secondary antioxidants) the freeperoxide radicals produced in the reduction of oxygen during operationand thereby improve the life and stability of the membrane andmembrane-electrode unit as described in JP2001118591 A2. The mode ofaction and molecular structure of such additives are described in F.Gugumus in Plastics Additives, Hanser Verlag, 1990; N. S. Allen, M. EdgeFundamentals of Polymer Degradation and Stability, Elsevier, 1992; or H.Zweifel, Stabilization of Polymeric Materials, Springer, 1998.

Nonlimiting examples of such additives are:

-   bis(trifluoromethyl) nitroxide, 2,2,-diphenyl-1-picrinylhydrazyl,    phenols, alkylphenols, sterically hindered alkylphenols such as    Irganox, aromatic amines, sterically hindered amines such as    Chimassorb; sterically hindered hydroxylamines, sterically hindered    alkylamines, sterically hindered hydroxylamines, sterically hindered    hydroxylamine ethers, phosphites such as Irgafos, nitrosobenzene,    methyl-2-nitrosopropane, benzophenone, benzaldehyde tert-butyl    nitron, cysteamine, melanines, lead oxides, manganese oxides, nickel    oxides, cobalt oxides.

Possible fields of use of the doped polymer membranes of the inventioninclude, inter alia, use in fuel cells, in electrolysis, in capacitorsand in battery systems. Owing to their property profile, the dopedpolymer membranes are preferably used in fuel cells.

The present invention also provides a membrane-electrode unit comprisingat least one polymer membrane according to the invention. For furtherinformation on membrane-electrode units, reference may be made to thespecialist literature, in particular the patents U.S. Pat. No.4,191,618, U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4,333,805. Thedisclosure of the abovementioned references [U.S. Pat. No. 4,191,618,U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4,333,805] in respect of thestructure and the production of membrane-electrode units and also theelectrodes, gas diffusion layers and catalysts to be selected isincorporated by reference into the present description.

In one variant of the present invention, membrane formation can becarried out directly on the electrode rather than on a support. Thetreatment according to step D) can in this way be correspondinglyshortened, since the membrane no longer has to be self-supporting. Sucha membrane or coated electrode is also provided by the presentinvention.

The polymerization/formation of the oligomers can also occur in step A)and the solution can be applied to the electrode by means of a doctorblade. Step C) can then be partly or entirely omitted.

The above-described variants and preferred embodiments also apply tothis case, so that they will not be repeated at this point.

The coating obtained in step D) has a thickness of from 2 to 3000 μm,preferably from 3 to 2000 μm, in particular from 5 to 1500 μm.

An electrode which has been coated in this way can be installed in amembrane-electrode unit which, if appropriate, has at least one polymermembrane according to the invention.

General Measurement Methods:

Measurement methods for IEC

The conductivity of the membrane depends strongly on the content of acidgroups expressed as the ion-exchange capacity (IEC). To measure theion-exchange capacity, a specimen having a diameter of 3 cm is stampedout and placed in a glass beaker containing 100 ml of water. The acidliberated is titrated with 0.1M NaOH. The specimen is subsequently takenup, excess water is dabbed off and the specimen is dried at 160° C. for4 hours. The dry weight, m₀, is then determined gravimetrically to aprecision of 0.1 mg. The ion-exchange capacity is then calculated fromthe consumption of 0.1M NaOH to the first titration end point, V₁ in ml,and the dry weight, m₀ in mg, according to the following formula:IEC=V ₁*300/m ₀Measurement Method for Specific Conductivity

The specific conductivity is measured by means of impedance spectroscopyin a 4-pole arrangement in the potentiostatic mode using platinumelectrodes (wire, 0.25 mm diameter). The distance between thecurrent-collecting electrodes is 2 cm. The spectrum obtained isevaluated using a simple model comprising a parallel arrangement of anohmic resistance and a capacitor. The specimen cross section of themembrane doped with phosphoric acid is measured immediately beforemounting of the specimen. To measure the temperature dependence, themeasurement cell is brought to the desired temperature in an oven andthe temperature is regulated by means of a Pt-100 resistance thermometerpositioned in the immediate vicinity of the specimen. After thetemperature has been reached, the specimen is maintained at thistemperature for 10 minutes before commencement of the measurement.

EXAMPLES

Stock Solution for the Preparation of PBI Membranes Sulfonated In Situ

938.6 g of polyphosphoric acid (83.4±0.5% of P₂O₅) were added to amixture of 26.948 g of isophthalic acid and 34.74 g of3,3′,4,4′-tetraaminobiphenyl in a 1.5 l flask equipped with nitrogeninlet and outlet and a mechanical stirrer. This mixture was heated at120° C. for 2 hours, at 150° C. for 3 hours and at 180° C. for 2 hours.The reaction solution was then heated at 220° C. and stirred for 14hours. The resulting 5% strength PBI solution in PPA was cooled to RTand used for producing the following sulfonated PBI membranes.

A small part of the solution was precipitated by means of water. Theprecipitated resin was filtered off, washed three times with H₂O,neutralized with ammonium hydroxide, then washed with H₂O and dried at100° C. and 0.001 bar for 16 hours. The inherent viscosity η_(inh) wasmeasured on a 0.4% strength PBI solution in 100 ml of 96% strengthH₂SO₄, giving a value of 1.52 dl/g.

Specimen 1: (PPA/1sPBI Membrane)

22.34 g of 85% strength phosphoric acid and 1.66 g of 96% strengthsulfuric acid were added to 100 g of the above-described 5% strength PBIstock solution in 113.6% of PPA at 220° C. over a period of 30 minutes.This solution was stirred at 220° C. for a further 4 hours. Theresulting sulfonated PBI solution in PPA was applied to a glass plate at220° C. by means of a preheated doctor blade (381 μm). A transparentmembrane was obtained. The membrane was then allowed to stand at RT for1 day, giving a self-supporting membrane.

Specimen 2: (PPA/2sPBI Membrane)

17.24 g of 85% strength phosphoric acid and 3.314 g of 96% strengthsulfuric acid were added to 100 g of the above-described 5% strength PBIstock solution in 113.6% of PPA at 220° C. over a period of 30 minutes.This solution was stirred at 220° C. for a further 4 hours. Theresulting sulfonated PBI solution in PPA was applied to a glass plate at220° C. by means of a preheated doctor blade (381 μm). A transparentmembrane was obtained. The membrane was then allowed to stand at RT for1 day.

Specimen 3: (PPA/3sPBI Membrane)

24.76 g of 85% strength phosphoric acid and 4.97 g of 96% strengthsulfuric acid were added to 100 g of the above-described 5% strength PBIstock solution in 113.6% of PPA at 220° C. over a period of 30 minutes.This solution was stirred at 220° C. for a further 4 hours. Theresulting sulfonated PBI solution in PPA was applied to a glass plate at220° C. by means of a preheated doctor blade (381 μm). A transparentmembrane was obtained. The membrane was then allowed to stand at RT for1 day.

Specimen 4: (PPA/4sPBI Membrane)

38.89 g of 85% strength phosphoric acid and 6.6288 g of 96% strengthsulfuric acid were added to 100 g of the above-described 5% strength PBIstock solution in 113.6% of PPA at 220° C. over a period of 30 minutes.This solution was stirred at 220° C. for a further 4 hours. Theresulting sulfonated PBI solution in 105.1% strength PPA was applied toa glass plate at 220° C. by means of a preheated doctor blade (381 μm).A transparent membrane was obtained. The membrane was then allowed tostand at RT for 1 day.

Specimen 5: (PPA/6sPBI Membrane)

41.22 g of 85% strength phosphoric acid, 19.333 g of 115 strengthpolyphosphoric acid and 9.943 g of 96% strength sulfuric acid were addedto 100 g of the above-described 5% strength PBI stock solution in 113.6%of PPA at 220° C. over a period of 30 minutes. This solution was stirredat 220° C. for a further 4 hours. The resulting sulfonated PBI solutionin PPA was applied to a glass plate at 220° C. by means of a preheateddoctor blade (381 μm). A transparent membrane was obtained. The membranewas subsequently allowed to stand at RT for 1 day. SO₃H Thickness IV Scontent content H₃PO₄ IEC IEC at RT at 120° C. Specimen [μm] [dl/g] [%][%] Content [meq/g] [eq/cm3] σ (S/cm) σ (S/cm) 1 210 1.14 4.16 14.414.23 138.56 2.5 0.092 0.064 2 231 1.16 6.22 21.5 15.71 153.01 3.210.083 0.082 3 186 1.1 8.49 29.4 20.16 196.36 3.16 0.099 0.054 4 184 1.148.95 30.6 19.78 192.66 3.14 0.09 0.089 5 218 1.14 9.16 31.7 23.08 224.763.22 0.096 0.092

1. A proton-conducting polymer membrane which is based on sulfonatedpolymers based on polymers comprising recurring benzimidazole units andis obtainable by a process comprising the steps A) mixing of one or morearomatic tetraamino compounds with one or more aromatic carboxylic acidsor esters thereof which contain at least two acid groups per carboxylicacid monomer, or mixing of one or more aromatic and/or heteroaromaticdiaminocarboxylic acids, in a polyphosphoric acid/sulfonating agentmixture to form a solution and/or dispersion, B) application of a layerusing the mixture from step A) to a support or an electrode, C) heatingof the sheet-like structure/layer obtainable according to step B) underinert gas at temperatures of up to 350° C., preferably up to 280° C., toform the polyazole polymer, D) treatment of the polymer layer producedin step C) in the presence of moisture at temperatures and for a timeuntil the membrane is self-supporting and can be detached from thesupport without damage.
 2. The membrane as claimed in claim 1,characterized in that aromatic tetraamino compounds used are3,3′,4,4′-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine,1,2,4,5-tetraaminobenzene, bis(3,4-diaminophenyl) sulfone,bis(3,4-diaminophenyl) ether, 3,3′,4,4′-tetraaminobenzophenone,3,3′,4,4′-tetraaminodiphenylmethane and3,3′,4,4′-tetraaminodiphenyldimethylmethane.
 3. The membrane as claimedin claim 1, characterized in that aromatic dicarboxylic acids used areisophthalic acid, terephthalic acid, phthalic acid, 5-hydroxyisophthalicacid, 4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid,5-aminoisophthalic acid, 5-N,N-dimethylaminoisophthalic acid,5-N,N-diethylaminoisophthalic acid, 2,5-dihydroxyterephthalic acid,2,5-dihydroxyisophthalic acid, 2,3-dihydroxyisophthalic acid,2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid,3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalicacid, 2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-dicarboxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid, 4-carboxycinnamic acid, or their C1-C20-alkyl esters orC5-C12-aryl esters, or their acid anhydrides or acid chlorides.
 4. Themembrane as claimed in claim 1, characterized in that aromaticcarboxylic acids used are tricarboxylic acids, tetracarboxylic acids ortheir C1-C20-alkyl esters or C5-C12-aryl esters or their acid anhydridesor their acid chlorides, preferably 1,3,5-benzenetricarboxylic acid(trimesic acid); 1,2,4-benzenetricarboxylic acid (trimellitic acid);(2-carboxyphenyl)iminodiacetic acid, 3,5,3′-biphenyltricarboxylic acid;3,5,4′-biphenyltricarboxylic acid and/or 2,4,6-pyridinetricarboxylicacid.
 5. The membrane as claimed in claim 1, characterized in thataromatic carboxylic acids used are tetracarboxylic acids, theirC1-C20-alkyl esters or C5-C12-aryl esters or their acid anhydrides ortheir acid chlorides, preferably benzene-1,2,4,5-tetracarboxylic acid;naphthalene-1,4,5,8-tetracarboxylic acid,3,5,3′,5′-biphenyltetracarboxylic acid; benzophenonetetracarboxylicacid, 3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid.
 6. The membrane as claimed inclaim 4, characterized in that the content of tricarboxylic acids ortetracarboxylic acids (based on dicarboxylic acids used) is from 0 to 30mol %, preferably from 0.1 to 20 mol %, in particular from 0.5 to 10 mol%.
 7. The membrane as claimed in claim 1, characterized in thatheteroaromatic carboxylic acids used are heteroaromatic dicarboxylicacids and tricarboxylic acids and tetracarboxylic acids in which atleast one nitrogen, oxygen, sulfur or phosphorus atom is present in thearomatic, preferably pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazinedicarboxylic acid, 2,4,6-pyridinetricarboxylic acid,benzimidazole-5,6-dicarboxylic acid, and also their C1-C20-alkyl estersor C5-C12-aryl esters, or their acid anhydrides or their acid chlorides.8. The membrane as claimed in claim 1, characterized in that thesulfonating agent used in step A) is selected from the group consistingof i) concentrated sulfuric acid (>95%), ii) chlorosulfonic acid, iii) acomplex of SO₃ with a Lewis base or other organic constituents, iv) anacyl or alkyl sulfate, v) an organic sulfonic acid and vi) mixtures of ito v.
 9. The membrane as claimed in claim 1, characterized in thataromatic and heteroaromatic diaminocarboxylic acids used arediaminobenzoic acid and its monohydrochloride and dihydrochloridederivatives.
 10. The membrane as claimed in claim 1, characterized inthat a polyphosphoric acid having an assay calculated as P₂O₅(acidimetric) of at least 83% is used in step A).
 11. The membrane asclaimed in claim 1, characterized in that a polymer which is based onpolymers comprising recurring benzimidazole units of the general formula(I) and/or (II) and/or (III) and/or (IV) and/or (V) and/or (VI) and/or(VII) and/or (VIII) and/or (IX) and/or (X) and/or (XI) and/or (XII)and/or (XIII) and/or (XIV) and/or (XV) and/or (XVI) and/or (XVI) and/or(XVII) and/or (XVIII) and/or (XIX) and/or (XX) and/or (XXI) and/or(XXII)

where the radicals Ar are identical or different and are each atetravalent aromatic or heteroaromatic group which can be monocyclic orpolycyclic, the radicals Ar¹ are identical or different and are each adivalent aromatic or heteroaromatic group which can be monocyclic orpolycyclic, the radicals Ar² are identical or different and are each adivalent or trivalent aromatic or heteroaromatic group which can bemonocyclic or polycyclic, the radicals Ar³ are identical or differentand are each a trivalent aromatic or heteroaromatic group which can bemonocyclic or polycyclic, the radicals Ar⁴ are identical or differentand are each a trivalent aromatic or heteroaromatic group which can bemonocyclic or polycyclic, the radicals Ar⁵ are identical or differentand are each a tetravalent aromatic or heteroaromatic group which can bemonocyclic or polycyclic, the radicals Ar⁶ are identical or differentand are each a divalent aromatic or heteroaromatic group which can bemonocyclic or polycyclic, the radicals Ar⁷ are identical or differentand are each a divalent aromatic or heteroaromatic group which can bemonocyclic or polycyclic, the radicals Ar⁸ are identical or differentand are each a trivalent aromatic or heteroaromatic group which can bemonocyclic or polycyclic, the radicals Ar⁹ are identical or differentand are each a divalent or trivalent or tetravalent aromatic orheteroaromatic group which can be monocyclic or polycyclic, the radicalsAr¹⁰ are identical or different and are each a divalent or trivalentaromatic or heteroaromatic group which can be monocyclic or polycyclic,the radicals Ar¹¹ are identical or different and are each a divalentaromatic or heteroaromatic group which can be monocyclic or polycyclic,the radicals X are identical or different and are each oxygen, sulfur oran amino group which bears a hydrogen atom, a group having 1-20 carbonatoms, preferably a branched or unbranched alkyl or alkoxy group, or anaryl group as further radical, the radicals R are identical or differentand are each hydrogen, an alkyl group or an aromatic group and n, m areeach an integer greater than or equal to 10, preferably greater than orequal to 100,

where the radicals R are identical or different and are each an alkylgroup or an aromatic group and n is an integer equal to or greater than10, preferably equal to or greater than 100, and has a sulfur content offrom 2 to 20% by weight (determined by means of elemental analysis), isformed in step C).
 12. The membrane as claimed in claim 1, characterizedin that the viscosity is adjusted by addition of phosphoric acid afterstep A) and before step B).
 13. The membrane as claimed in claim 1,characterized in that the treatment of the membrane in step D) iscarried out at temperatures from above 0° C. to 150° C., preferably attemperatures in the range from 10° C. to 120° C., in particular fromroom temperature (20° C.) to 90° C., in the presence of moisture orwater and/or steam.
 14. The membrane as claimed in claim 1,characterized in that an electrode is selected as support in step B) andthe treatment according to step D) is such that the membrane formed isno longer self-supporting.
 15. An electrode provided with aproton-conducting polymer coating which is based on polymers comprisingrecurring benzimidazole units and is obtainable by a process comprisingthe steps A) mixing of one or more aromatic tetraamino compounds withone or more aromatic sulfocarboxylic acids or esters thereof whichcontain at least two carboxylic acid groups and one sulfonic acid groupper carboxylic acid monomer, or mixing of one or more aromatic and/orheteroaromatic sulfonated diaminocarboxylic acids, in a polyphosphoricacid to form a solution and/or dispersion, B) application of a layerusing the mixture from step A) to an electrode, C) heating of thesheet-like structure/layer obtainable according to step B) under inertgas at temperatures of up to 350° C., preferably up to 280° C., to formthe polyazole polymer, D) treatment of the membrane formed in step C) inthe presence of moisture at temperature and for a time sufficient forthe layer to have sufficient strength for use in fuel cells.
 16. Amembrane-electrode unit comprising at least one electrode and at leastone membrane as claimed in one or more of claims 1 to
 14. 17. Amembrane-electrode unit comprising at least one electrode as claimed inclaim 16 and at least one membrane as claimed in one or more of claims 1to
 14. 18. A fuel cell comprising one or more membrane-electrode unitsas claimed in claim 16 or 17.